1. Field of the Invention
This invention relates to a method and an apparatus for non-consumable electrode type automatic arc welding, in which an arc length is automatically adjusted to an optimum value.
2. Description of the Prior Art
In non-consumable electrode type automatic arc welding, it is imperative that the length of an arc be maintained constant, despite the type of movement of the torch or shape of the weld grove, to thereby obtain uniform melting of a filler wire which is automatically fed, such that uniform quality of the weldment is obtained. More particularly, it is desired when utilizing non-consumable electrode type automatic arc welding that the torch is caused to weave or oscillate in all positions in order to obtain flat beads and to minimize internal defects. However, unless the arc length is accurately controlled, there will result variation in the arc length due to the irregularity of surface conditions of a weld groove and that of an underlaid bead. Variation in the arc length can also result due to variation in the spacing between the torch and workpiece being welded during the weaving operation of the torch. Thus, in the event that the arc length is too short, there will arise the possibility of short-circuiting between a non-consumable electrode and a workpiece being welded, damage to the electrode or deposition of the material of the electrode into the weld. In addition, due to variation in the arc length, i.e., variations in current density of an arc and in the cross-sectional area of an arc relative to that of a molten pool or a weld puddle, stable penetration of the weld may not be obtained, and uniform bead formations may not be obtained resulting from non-uniform formation of puddles of the weld. Still further, with non-consumable electrode type automatic arc welding, in which a filler wire is automatically fed, a minor variation in arc length will lead to a variation in the melting rate of a filler wire, such that there will result non-uniform formation of beads or immersion of the filler wire into a weld puddle due to an unbalanced feed rate of the filler wire with respect to its melting rate. The failure to effect normal transfer or shifting of the molten metal in droplet form from a filler wire to a weld puddle or molten pool, may result due to too high a feed rate of the filler wire in the cases of overhead or vertical position welding.
It follows that in the case of non-consumable electrode type automatic arc welding, particularly when weld weaving in all positions is is used, it is highly desirable to maintain the arc length accurately constant, and thus such a control system is urgently desired.
To this end, there has been proposed in a Japanese Patent Publication (1969 - 1822), that an arc voltage is detected and amplified so as to move an electrode in a vertical direction, thereby maintaining the arc length constant. This attempt is to provide a control system in which a proportional characteristic affording a dead zone of an arc voltage is maintained between the arc voltage and the voltage fed to the electric motor adapted to drive a welding electrode, thereby eliminating the hunting of the electric motor.
With such prior art control systems, there result three disadvantages in that the stabilized point of an arc voltage returning when the arc voltage is lowered and the stabilized point returning when the arc voltage is raised will necessarily lead to the stoppage of the electric motor in different positions within such a range of the arc voltage, in that the electric motor fails to respond to such a voltage, or in that the stabilized points will not remain at a given point due to the variations in an arc voltage and due to the fact that the control distance of the electric motor varies with the varying impressed voltages. In addition, with a control system wherein such a stabilized operating point is not definite with respect to its position within the aforesaid non-responsive range or dead zone, difficulties are encountered with adjustment of the set arc voltage. Moreover, the responding capability of a control system to the variation in arc voltage will be lowered to an extent corresponding to the aforesaid non-responsive range or dead zone.
Thus, such an arc control system in non-consumable electrode type automatic arc welding fails to meet the requirements of accurate welding, all position welding and welding by weaving or oscillating a torch, as are used for welding of high alloy steels, mild steels and low alloy steels. Also, such a system cannot obtain uniform melting of a filler wire with resulting high quality welds which require a well controlled arc length.